January 23rd 2008

XORLY? Losing your GPG private key safely

It's quite convenient to have your GPG private key on your desktop, laptop and USB stick.. but if one of these is stolen or lost, you are pretty much obliged to issue a revocation.

Now, RAID-5 has this wonderful property whereby it is impossible to recover data from an array of size n if you have (at most) n-2 elements. So, one idea would be to create a small loopback device -based array that you can spread around your nice stealable items - you then use your key by combining >= n-1 of them.

..Or Eve must steal n-1 items before she can reconstruct your secret key. You can recover the "missing data" from single lost device, obviously.

There are almost certainly cooler algorithms for this, but none that are in the Linux kernel. Some scriptage that seems to work for me:


set -eu


create() {
   for NUM in $(seq $NUMITEMS);
       dd if=/dev/zero of=$NUM.img bs=1M count=$(echo "$SIZE/($NUMITEMS-1)" | bc)
       losetup /dev/loop$NUM $NUM.img

   sudo mdadm --create -l5 -n$NUMITEMS /dev/md0 $(seq -f "/dev/loop%g" $NUMITEMS)
   sudo mkfs.ext3 /dev/md0

stop() {
   sudo mdadm --stop /dev/md0

   for NUM in $(seq 1 $NUMITEMS);
       sudo losetup -d /dev/loop$NUM || true

start() {
   # Collect array elements from argv
   for ELEM in $ELEMENTS;
       NUM=$(( $NUM + 1 ))
       sudo losetup /dev/loop$NUM "$ELEM"
   sudo mdadm --assemble --run /dev/md0 $(seq -f "/dev/loop%g" $NUM)

recreate() {
   # Assume already running
   dd if=/dev/zero of=$NUMITEMS.img bs=1M count=$(echo "$SIZE/($NUMITEMS-1)" | bc)
   losetup /dev/loop$NUMITEMS $NUMITEMS.img

   sudo mdadm --add /dev/md0 $NEW
   sudo mdadm --misc --wait /dev/md0

(This proof-of-concept script is deliberately broken: you'll have to write the argument handling yourself and convince yourself you don't have anything important at /dev/loop* or /dev/md0 or /. It might also make sense to only use the array in read-only mode, or you will have syncing issues.)

Ingenious learners may wish to construct other scenarios with other RAID levels; 5-device RAID-6 would only require 3 elements present to function (or to be stolen by Eve, naturally).

Not that I'm actually doing this: it's almost certainly as stupid as it sounds, I just haven't thought of a convincing reason yet.

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